Measurements of four seawater inorganic carbon system parameters—pH, carbon dioxide fugacity (fCO2) or partial pressure of CO2 (p CO2), total dissolved inorganic carbon (DIC), and total alkalinity (TA)—are essential for carbon cycle investigations on both global and local scales. Both observational and modeling efforts rely on high quality inorganic carbon data from field measurements. Extensive efforts have been devoted to improving methodologies and instruments for determination of carbon parameters in seawater.
In standard conventional methodologies, the four core parameters of the seawater inorganic carbon system are measured using diverse instrumentation (e.g. potentiometry, spectrophotometry, gas chromatography, non-dispersive infrared analysis, and coulometry). Recent advances in technology and materials have prompted many researchers to adapt these diverse methodologies for use in unattended in-situ devices and autonomous underway systems without compromising precision and accuracy relative to standard methods (e.g. for in-situ pCO2, underway pCO2, underway pH, in-situ pH, underway DIC and underway TA).
It is preferable for all four parameters of the seawater inorganic carbon system are measured simultaneously and continuously with high temporal resolution, and with high precision and accuracy. Although any two of the four parameters are sufficient to fully characterize the inorganic carbon system via thermodynamic calculations, additional parameters are required to ensure internal consistency of measurements and evaluate the thermodynamic characterizations that relate various CO2 system parameters.
Among all available methodologies for measurements of inorganic carbon species in seawater, spectrophotometric methods are especially promising because they can be used to unify measurements of different parameters and achieve simultaneous multi-parameter measurements at relatively lowcost. Moreover, spectrophotometric methods have many advantageous attributes for measuring inorganic carbon species in seawater: high sensitivity, good stability and selectivity, simplicity, and low rates of sample and reagent consumption.
Spectrophotometric pH measurements using absorbance ratios at multiple wavelengths have long been utilized to obtain precise (±0.0004) discrete shipboard measurements of seawater pH. As such, spectrophotometric pH measurements are approximately an order of magnitude more precise than potentiometric pH measurements. As an additional advantage relative to potentiometric measurements, at-sea spectrophotometric pH measurements do not require calibration subsequent to laboratory characterizations of each indicator's molecular properties as a function of temperature, salinity, pressure and ionic strength. Since pH measurements require very low indicator concentrations (<2_M), pH perturbations due to indicator addition are quite small. Underway spectrophotometric pH measurements can achieve precisions close to those of discrete shipboard spectrophotometric measurements.
Researchers have also developed spectrophotometric sensors for oceanic p CO2 measurements. These sensors have been deployed for measurements on moorings with reported precisions (1-2_atm) close to those obtained in shipboard measurements using a CO2 gas equilibrator with nondispersive infrared analysis. Spectrophotometric p CO2 measurements are generally based on the same principle as those utilized in spectrophotometric pH measurements.
A membrane-optical cell containing a sulfonephthalein indicator functions as a traditional spectrophotometric cell and a CO2 equilibrator. Water samples surround the optical cell but do not have direct contact with the internal indicator solution. CO2 in water samples equilibrates with the cell's internal indicator solution. The resulting pH of the internal solution is measured by a spectrophotometer connected to the cell with optical fibers. The optical cell can be made of polytetrafluoroethylene (PTFE) or silicone, both of which are permeable to CO2 molecules.
Teflon AF 2400 is ideal for spectrophotometric p CO2 measurements because it is highly permeable to CO2 gas molecules, and can be used as a long pathlength liquid core waveguide (LCW). Long pathlengths can improve detection sensitivity, and high CO2 permeability can reduce equilibration times. Byrne et al. described spectrophotometric DIC measurements using Teflon AF 2400 tubing as both an optical cell and a gas permeable membrane. (see R. H. Byrne, X. Liu, E. Kaltenbacher, K. Sell, Anal. Chim Acta 451 (2002) 221; which is incorporated herein by reference). The method is similar to spectrophotometric pCO2 measurement except that water samples are acidified, converting all carbonate species to CO2. Spectrophotometric pH measurements are then used to determine DIC as total CO2 after equilibration across the wall of the LCW.
The procedure is quite simple compared to the coulometric method, and has a comparable precision (1-2 μmol kg−1) and accuracy (5 μmol kg−1). The reported response time was less than 15 min, which can be further improved by selection of thinner-wall and smaller-diameter capillary tubing. Spectrophotometric DIC measurements are also field portable and easily adapted to in-situ analysis.
What is needed, however, is an autonomous multi-parameter flow-through CO2 system for simultaneous measurement of surface seawater pH, fCO2, and DIC.